1. Field of the Invention
The present invention relates to control of a total energy amount irradiated to a sensitive subject, and more particularly to control of a total irradiated energy amount in the case where the energy amount from an energy generating source of pulse oscillation type is changed over time. The present invention relates to, for example, an energy amount control device suitable for exposure amount control in an exposure apparatus using a pulse laser like an excimer laser as light for exposure.
2. Related Background Art
Recently, in exposure apparatus used in the photolithography process for manufacture of semiconductor devices, the minimum pattern size of circuits has become finer with a higher degree of integration of the devices. There have also been developed exposure apparatus using an excimer laser as a light source for exposure in place of a mercury lamp mainly used in the past. It is general that the excimer laser and a body of the exposure apparatus are connected by an interface cable such as an electric wire or optical fiber to emit a laser beam in accordance with the sequence of a main computer in the exposure apparatus body. Interfacing signals includes, for example, signals indicative of emission trigger, start of high-voltage charging, start and stop of oscillation, etc. which are transmitted from the exposure apparatus body to the excimer laser, and signals indicative of completed state on standby for oscilation, internal shutter position, interlock under operation, etc. which are transmitted from the excimer laser to the exposure apparatus body.
In this type exposure apparatus, illuminance may vary on the exposure surface because there occur on an exposure surface (reticle or wafer) a regular interference pattern due to coherent laser beams and an irregular interference pattern (speckle) due to superposition of multiple rays with different phases caused by flaws, dust pieces, surface defects or the like in an illumination optical system, along with variations on the order of .+-.10% per pulse of a laser beam. The above interference patterns, in particular, the regular interference pattern, greatly affects control of a pattern line width in the photolithography process. It has therefore been proposed to smooth the regular interference pattern and speckles (hereinafter referred to collectively as interference pattern) by using a method equivalent to one disclosed in U.S. Pat. No. 4,619,508 or Japanese Patent Laid-Open No. 1-259533 (corresponding U.S. patent Ser. No. 322,207, filed on Mar. 13, 1989), for example. Smoothing (conversion into an incoherent state) of the interference pattern disclosed in the above reference is carried out such that the laser beam is moved one-dimensionally or two-dimensionally (raster scan) at a constant period by a vibrating mirror (such as a galvano-mirror or polygon mirror) to spatially reduce a degree of coherency. Specifically, an incident angle of the laser beam upon illuminance equalizing means (optical integrator) is changed per pulse to move the interference pattern on a reticle for finally smoothing the interference pattern, namely, increase a degree of illuminance uniformity. At this time, a plurality of pulses are irradiated in synchronism with a one-dimensional or two-dimensional scan by the vibrating mirror. Since an oscillation pulse width of the excimer laser is usually very short on the order of 20 nsec, the vibrating mirror looks like still, as it were, during one pulse of the excimer laser even if it is vibrated at about 10 Hz.
Accordingly, to achieve smoothing (equalizing of illuminance) of the interference pattern by plural pulses and desired control accuracy of the exposure amount, it is important that (1) the energy amount per pulse is kept substantially constant during exposure, and (2) the target exposure amount is obtained with the number of pulses integer times as many as a half period of mirror vibrations in the one-dimensional or two-dimensional scan of the vibrating mirror.
Meanwhile, a laser light source shows a reduction in laser density not only in short term but also in long term. Such a reduction in laser density is significant in gas lasers; i.e., the output power is reduced due to deterioration of a gas mixture of active media sealed off within a chamber. In an excimer laser, for example, a gas mixture consisted of three types of gases, i.e., halogen gas such as fluorine, inert gas such as krypton or argon, and rare gas such as helium or neon, are sealed off within a laser chamber so that the halogen gas and the inert gas are reacted with each other under discharge in the chamber to emit a laser beam. However, as the laser beam is emitted repeatedly, the concentration of the halogen gas is reduced and so is pulse energy of the laser beam, because the halogen gas may couple with impurities generated in the chamber or deposit on the inner wall of the chamber. Generally, when the relation between the applied voltage (or the charged voltage) of an excimer laser light source and the energy amount of an exposure pulse emitted under that applied voltage is changed over time, for example, when the output power of the laser light source is lowered due to deterioration of a gas mixture of active media (e.g., reduction in the concentration of halogen gas), the desired oscillation energy amount cannot be obtained even if the applied voltage corresponding to the pulse energy amount to be next emitted is determined from the above relation.
A laser light source is usually operated in an applied voltage constant mode or an energy amount constant mode. In the case where the output power is lowered due to reduction in the concentration of halogen gas or the like in the energy amount constant mode, this power loss has been conventionally compensated for by receiving part of a laser beam to detect the energy amount, and feed-backing the detected value to the applied voltage in such a manner as to gradually increase the applied voltage between two electrodes in the chamber. Further, since the applied voltage has an upper limit, it has been required that when the applied voltage reaches the upper limit, halogen injection (HI) is effected to return the concentration of halogen gas to a proper value for reducing the applied voltage. But, with the HI operation repeated, an amount of impurities in the laser chamber is increased. In spite of carrying out the HI operation, therefore, the halogen gas couple with the impurities in more amount and the applied voltage is raised up at a faster rate due to a reduction in the gas concentration. As a consequence, the period of the HI operation is gradually shortened and, though the applied voltage is near the upper limit, the pulse energy amount will be gradually reduced. Accordingly, when the HI operation has failed to develop an effective result or when the pulse energy amount has reduced down to a predetermined value, it has been also required to partially replace three types of mixture gases, that is, so-called partial gas replacement (PGR). Until the PGR operation is required again, the HI operation is repeated in a like manner. The HI and PGR operations have been substantially automatically performed by commands from a control processor in the excimer laser light source.
With the conventional exposure apparatus as explained above, however, although the oscillation energy amount of substantially constant can be always obtained by oscillating the excimer laser light source in the energy amount constant mode, it is impossible to produce the desired oscillation energy amount (or the applied voltage corresponding to the oscillation energy amount) throughout a predetermined range of energy amount control. Stated otherwise, the oscillation energy amount cannot be precisely or finely performed per pulse and the exposure amount cannot be controlled with high precision, resulting in the problem such as reduced yield. In addition, since the HI and PGR operations are exclusively controlled on the laser light source side alone, it may well be that when wafers are exposed using the step and repeat technique, if the HI or PGR operation is carried out in an asynchronous manner during exposure of one shot area (that usually requires more than several tens pulses), exposure of that shot and further the next shot are damaged to a large extent.